Poly(isobutylene-co-isoprene), or IIR, is a synthetic elastomer commonly known as butyl rubber which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (1-2 mole %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, usually not more than 2.5 mol %, of a conjugated multiolefin. Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a diluent and a Friedel-Crafts catalyst as part of the polymerization initiator. This process is further described in U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295.
Halogenation of this butyl rubber produces reactive allylic halide functionality within the elastomer. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company © 1987), particularly pp. 297-300.
In the production of halogenated butyl rubber, one crucial step is the preparation of a rubber solution in a non-halogenated organic solvent, for example hexane, (called rubber cement), starting from rubber crumbs in water.
The current state of art for forming a rubber cement involves several stages where the rubber slurry which typically comprises 5-20 wt % rubber and 80-95 wt % water) is passed through a dewatering screen to remove the bulk water, then subsequently transferred to a pre-dissolving drum through a rotary valve where it is mixed with hexanes. The swelled rubber is then transferred to the dissolving drum and the surge drum prior to halogenation.
There are several disadvantages associated with such a cement forming process, for example, the commonly used surge drums are open to the atmosphere, the rotary valve on the pre-dissolving drum is a significant source of hexane emissions, the dewatering screens and rotary valve may have process and mechanical reliability issues, and more importantly, the existing system is not suitable for use with certain non-halogenated organic solvents having a lower boiling point than hexane, such as isopentane or n-pentane, due to excessive emissions.
Efforts to improve the current process and improve mechanical reliability have been made. For example, a process is known, according to Russian Patent 2320672 C1, wherein chlorinated butyl rubber is produced by mixing a 3-5%-aqueous dispersion of butyl rubber with a hydrocarbon solvent, dissolving butyl rubber at 20-60° C., separating the aqueous layer, and reacting a 10-15%-butyl rubber solution with tert-butylhypochlorite taken in an amount of 8% per rubber. The process is carried out in one or a series of reactors at 10-50° C., followed by neutralization of excess tert-butylhypochlorite with aqueous sodium sulfite and sodium hydroxide solutions until tert-butylhypochlorite is completely decomposed and the aqueous phase has a pH of 2-3. The method further involves separation of the phases, washing the rubber solution with water, additionally treating the chlorinated butyl rubber solution with an aqueous sodium hydroxide solution to a pH of 7-8, and isolating and drying the rubber. The method provides improved separation of the hydrocarbon and aqueous phases and decreased content of inorganic substances in the product. However, there are several limitations to the process disclosed. First, it specifically deals with chlorobutyl rubber. Secondly, it refers to a commercially impractical range of 3-5% aqueous dispersion of butyl rubber. Thirdly, it discloses a vertical mixing vessel and a vertical separator configuration, which may be detrimental to the phase separation by not providing adequate interface area and/or forming a difficult to separate emulsion. In addition, the rubber may mat at the interface between phases and thereby plug the separator.
As a result, there remains a need to develop alternative approaches to improve the process, including widening the applicability of the butyl and butyl-like rubbers to be processed and the organic solvents to be used, to improve the reliability and the efficiency of the process, and to reduce solvent emission.